Like what has been said already, Brian was well-trained theory and lesson wise, and he also has a tremendous amount of natural talent and ability, both musically and with the instrument. He also started, musically, before the guitar with singing as a youngster and also playing a horn (the euphonium). So, he already had some musical training before seriously studying the guitar. Both may have given him a sense of melody and phrasing that only helped to shape his development as a guitarist and musician.
All this, however, doesn't take away from the hard work and effort that he put into his craft. The results speak for themselves, but I'm often also astonished by what he was/is still able to do on the instrument, but especially at such a young age. He's also pretty good at composing his own music too, and that's something that separates a great musician from a great guitarist; of which he is both.
By the way the gentleman that Brian took lessons from was Ray Gogarty, a jazz guitarist and legendary musician in his own right. On this or the old B.S. forum's archives, there was a whole topic with more info. about Ray Gogarty, including a great photo of him with several other jazz guitar legends.
Capt. Jones